Plaid Cymru will join forces with the Greens and the SNP to make a last ditch attempt to derail the so-called “bedroom tax” which will hit Wales harder than any other part of the UK.

The parties want the UK Government to make final hour changes to the controversial policy before it comes into force in April. Under this welfare reform, people will see a cut in their housing benefit if they are considered to have a spare bedroom in their council or housing association home.

Caroline Lucas, the UK’s first Green MP said: “I think there is a growing momentum from ordinary people feeling outraged about this.”

The UK Government’s own estimate is that 46% of Welsh housing benefit claimants will be affected, compared to a Great Britain average of just 31%. This represents a weekly loss of £12 per claimant.

Across Great Britain, 68% of couples under 60 without children are due to be affected, losing on average £16 a week.

Last week the chief executives of Carers UK, the MS Society, Mencap, Macmillan Cancer, Disability Rights UK, Carers Trust and Contact a Family, expressed deep concerns about the impact of the policy on disabled people and families caring unpaid for ill or disabled loved ones.

Concerns were voiced yesterday that people may struggle to find smaller accommodation.

Nick Bennett of Community Housing Cymru, the membership body for housing associations in Wales said: “The lack of mobility in the social housing sector is not a product of tenants needlessly under-occupying larger homes, but rather the log-jam created by a national shortage of affordable homes, and in particular, one and two bedroom properties.”

Plaid Arfon MP Hywel Williams said areas such as his North Wales constituency that have high numbers of students and holiday homes were under severe housing pressure and would be particularly badly affected.

He said: “This is a policy throughout the UK. It takes absolutely no notice of local circumstances like that.”

Mr Williams said he did not think the policy had been properly thought through.

Hoping that Liberal Democrats will oppose the policy, he said: “I think there’s a particular challenge also for the Liberal Democrats on this. A lot of them are uneasy with the direction the Government are taking anyway and this seems to me to be yet another unkind cruel cut that some of them mind find difficult to live with, although who knows?”

A spokesman for the Department for Work and Pensions said: “We are providing councils in Wales with £6.1m this year to support people and vulnerable groups who might be affected by these changes. We need to ensure a better use of social housing when thousands of tenants are living in overcrowded homes and many more are on housing waiting lists across the country.”

The UK Government argues the housing benefit bill has now hit £23bn and social housing tenants living in homes that are larger than their needs should make a contribution towards their rent or move to different accommodation.

It suggests that people who do not move can start work or do more hours or take in a lodger.

The department understands there are nearly one million “extra” bedrooms that are paid for by housing benefit at a cost of up to £500m a year.

He said: “Parents of soldiers serving in our armed forces, divorced parents whose kids come to stay and disabled people who need extra space because of their condition will all pay more for their homes. Even if this policy were fair, it is economically incoherent as it will force local authorities to relocate families to smaller, more expensive properties in the private rented sector or face increased arrears in areas where no such alternative properties exist.”